#include <string.h>
#include <stdlib.h>
#include <stddef.h>

struct big{
    unsigned long long ull_a;
    unsigned long long ull_b;
    unsigned long long ull_c;
    int si_d;
    int si_e;
};

void func(void){
    size_t skip = offsetof(struct big ull_b);
    unsigned char *ptr = (unsigned char *)malloc(sizeof(struct big));
    if (ptr == NULL){
        /* Handle malloc() error */
    }
    memset(ptr + skip,0,sizeof(struct big) - skip);
    /* ... */
    free(ptr);
    ptr = NULL;
}